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Journal of Catalysis xxx (xxxx) xxx

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Journal of Catalysis

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Atomically dispersed Ptn+ species as highly active sites in Pt/In2O3catalysts for methanol synthesis from CO2 hydrogenation

https://doi.org/10.1016/j.jcat.2020.06.0180021-9517/� 2020 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (C. Li).

Please cite this article as: Z. Han, C. Tang, J. Wang et al., Atomically dispersed Ptn+ species as highly active sites in Pt/In2O3 catalysts for methanol syfrom CO2 hydrogenation, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2020.06.018

Zhe Han a,b, Chizhou Tang b, Jijie Wang b, Landong Li a, Can Li b,⇑a School of Materials Science and Engineering & National Institute for Advanced Materials, Nankai University, Tianjin 300350, Chinab State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 April 2020Revised 13 June 2020Accepted 15 June 2020Available online xxxx

Keywords:CO2 hydrogenationIn2O3Atomically dispersed Ptn+ species

Hydrogenation of CO2 into methanol using the H2 produced from renewable energy is a promising wayfor carbon capture and utilization. Therefore, the methanol synthesis catalysts with high methanol selec-tivity are highly desired. In this work, we found that the methanol selectivity of In2O3 catalyst can be sig-nificantly enhanced by introducing a small amount of Pt. Methanol selectivity can be increased from72.2% (In2O3) to 91.1% (0.58 wt.% Pt/In2O3) at 220 �C. The introduced Pt atoms are doped into theIn2O3, forming atomically dispersed Ptn+ species, most of which are stable under working conditions. Itis proposed that the atomically dispersed Ptn+ species are responsible for the enhanced methanol selec-tivity, while Pt nanoparticles on In2O3 mainly boost the reverse water-gas shift reaction (CO2 + H2 ? CO +H2O).

� 2020 Elsevier Inc. All rights reserved.

1. Introduction Cu/ZnO/Al O /ZrO [19], Pd/ZnO [20], Au/CeO /TiO [21], and Ni -

The increasing atmospheric CO2 concentration caused byhuman activity has become a worldwide concern since CO2 is con-sidered to be responsible for global climate change and ocean acid-ification [1–3]. Several routes, such as CO2 sequestration andconversion of CO2 into chemicals, have been proposed for thereduction of carbon emission. Utilizing CO2 as a carbon resourceto produce valuable chemicals is more viable. CO2 can be convertedto various chemicals, including methane, olefins, aromatics, andalcohols [4–11]. Methanol is an ideal product since it is a platformmolecule for both energy storage and chemical synthesis [12].There are two major obstacles to be overcome to reduce CO2 emis-sion by converting CO2 into methanol. First, the H2 should be pro-duced utilizing renewable energy instead of fossil resources [13].Second, highly efficient catalysts for selective hydrogenation ofCO2 into methanol need to be developed.

Ternary Cu/ZnO/Al2O3 catalyst has been used for the methanolsynthesis from syngas (CO and H2). This catalyst has also beentested for CO2 hydrogenation [14]. However, Cu/ZnO/Al2O3 isactive in both methanol synthesis reaction and reverse water-gasshift (RWGS) reaction, leading to low methanol selectivity. UnlikeCO hydrogenation, a lot of water is generated during CO2 hydro-genation, causing the sintering of Cu and deactivation of the cata-lyst. Other catalysts, such as Cu/ZnO/Ga2O3 [15], Cu/ZrO2 [16–18],

2 3 2 x 2 5

Ga3/SiO2 [22] have also been tested with this reaction, confrontingwith similar problems.

Recently, catalysts based on metal oxides have attractedresearch attention. For example, ZnO-ZrO2 solid solution catalystshows excellent performance in CO2 hydrogenation for methanolsynthesis. This catalyst achieves methanol selectivity up to 86–91% and is very stable under working conditions [23]. Densityfunctional theory (DFT) calculations suggest that the high metha-nol selectivity is attributed to the simultaneous activation of H2and CO2 on the neighboring Zn2+ and Zr4+ sites [23]. CdZrOx andGaZrOx solid solution catalysts also show good catalytic activityand selectivity to methanol for this reaction [24]. In addition tothe ZrO2 based solid solution catalysts, In2O3 was also predictedto be active in CO2 hydrogenation and possess high selectivity tomethanol based on DFT calculations, which have been furtherdemonstrated by experiments [25–29].

Although In2O3 catalyst shows higher methanol selectivitycompared to Cu/ZnO/Al2O3, its activity is relatively low. SupportingIn2O3 on other supports has been investigated to disperse In2O3and improve the activity. However, only a few supported In2O3 cat-alysts such as In2O3/ZrO2 show some limited improvement [28]. Asecond active metal such as Co [30] and Ni [31] has also been intro-duced into In2O3 to improve the performance. Precious metals arewidely used to enhance the catalytic hydrogenation performanceof oxide-based catalysts. In this regard, it is reported that introduc-ing Pd nanoparticles prepared separately by a peptide-assisted method into In2O3 can improve the methanol synthesis

nthesis

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2 Z. Han et al. / Journal of Catalysis xxx (xxxx) xxx

performance [32]. In another article, the reactivity of Pd in In2O3 isfound to be related to its state. The low-nuclearity Pd clustersformed in-situ from the isolated Pd atoms in the In2O3 latticeimprove the methanol synthesis performance without boostingthe detrimental RWGS reaction, while Pd nanoparticles mainlypromote the RWGS reaction [33]. In addition to Pd, Pt is alsoreported to improve the performance of In2O3 in CO2 hydrogena-tion with an inferior improvement compared to Pd in Ref. [33].However, this article only focuses on the different effects of low-nuclearity Pd clusters and Pd nanoparticles. The detailed state ofPt and the mechanism of the Pt enhancement have not been inves-tigated. Herein, we investigated the effect of Pt on the performanceof In2O3 in the CO2 hydrogenation. We found that the addition of asmall amount of Pt into In2O3 through co-precipitation couldincrease both the methanol selectivity and the activity in CO2hydrogenation. It is affirmed that atomically dispersed Ptn+ speciesis responsible for the improved methanol selectivity while Ptnanoparticles mainly boost the RWGS reaction.

2. Experimental section

2.1. Catalyst preparation

In2O3 was prepared following a published method with somemodifications [34]. A solution of aqueous NH4OH (80 mL, 25.0–28.0%, AR, Sinopharm) in ethanol (240 mL, AR, Damao chemicalreagent) was added to a solution of In(NO3)3�xH2O (27.5 g,99.99%, Macklin) in a mixture of water (80 mL) and ethanol(240 mL) under vigorous stirring at room temperature. The result-ing slurry was immersed into a preheated water bath at 80 �C andstirred for 1 h before cooled to room temperature. The precipitatewas separated by centrifugation, washed with water, and dried at70 �C in air. The resulting solid was calcinated in air for 3 h at450 �C to obtain the final oxide sample.

Pt/In2O3 with Pt loading of 0.03–0.58 wt.% was prepared by co-precipitation. For 0.58 wt.% Pt/In2O3, a solution of (NH4)2CO3(15.6 g, AR, Sinopharm) in water (200 mL) was added to a solutionof (NH4)2PtCl6 (0.7 g, Beijing chemicals factory) and In(NO3)3�xH2O(28.6 g, AR, Macklin) in water (300 mL) under vigorous stirring atroom temperature. The resulting slurry was immersed into a pre-heated water bath at 70 �C and stirred for 2 h before cooled toroom temperature. The precipitate was separated by centrifuga-tion, washed with water, and dried at 70 �C in air. The resultingsolid was calcinated in air for 3 h at 450 �C to obtain the final cat-alyst sample. For Pt/In2O3 with Pt loading between 0.03 wt.% and0.34 wt.%, the amount of (NH4)2PtCl6 and (NH4)2CO3 was changedaccordingly while other procedures were kept as the same.

The 2.50 wt.% Pt/In2O3 and 0.65 wt.% Pt/SiO2 were prepared bywet impregnation, using platinum nitrate solution (Aladdin, Pt,18.02%) as the precursor and In2O3 or SiO2 (Aladdin, 99.5%) as sup-ports. The mixture containing Pt precursor, water, and support wasdried using a rotary evaporator and calcinated in air for 3 h at400 �C to obtain the final catalyst.

2.2. Catalyst evaluation

CO2 hydrogenation was performed using a quartz-lined stain-less steel fixed-bed reactor. The catalyst (0.1 g) and quartz sand(0.4 g) were evenly mixed and packed in the reactor. All the cata-lysts were used directly without any pretreatment. The reactanthas the composition of CO2/H2/Ar = 24/72/4 (molar ratio). Thepressure of the reactor was controlled by a high-temperatureback-pressure regulator. Products were analyzed online using gaschromatography (Agilent 7890B). CO2, Ar, and CO were separatedby coupled Porapak N and MolSieve 5A packed columns and quan-

Please cite this article as: Z. Han, C. Tang, J. Wang et al., Atomically dispersed Ptfrom CO2 hydrogenation, Journal of Catalysis, https://doi.org/10.1016/j.jcat.202

tified by a thermal conductivity detector (TCD). Methane, dimethylether, and methanol were separated by an HP-PLOT Q capillary col-umn and quantified by a flame ionization detector (FID). The wholearea between the reactor outlet and GC inlet, including the back-pressure valve, transfer lines, and GC sampling valves, was keptat 150 �C to avoid the condensation of the products.

The CO2 conversion (X) and product selectivity (S) were calcu-lated using the following equations:

X CO2ð Þ ¼ n CH4ð Þ þ n CH3OCH3ð Þ � 2þ n CH3OHð Þ þ n COð Þn CH4ð Þ þ n CH3OCH3ð Þ � 2þ n CH3OHð Þ þ n COð Þ þ nðCO2Þ

S CH3OHð Þ ¼ nðCH3OHÞn CH4ð Þ þ n CH3OCH3ð Þ � 2þ n CH3OHð Þ þ n COð Þ

S CH4ð Þ ¼ nðCH4Þn CH4ð Þ þ n CH3OCH3ð Þ � 2þ n CH3OHð Þ þ n COð Þ

S COð Þ ¼ nðCOÞn CH4ð Þ þ n CH3OCH3ð Þ � 2þ n CH3OHð Þ þ n COð Þ

where n(P) was the molar flow rate of P in the reactor outlet calcu-lated from peak areas on the chromatogram and relative sensitivefactors.

2.3. Catalyst characterization

Powder X-ray diffraction (PXRD) patterns were recorded using aRigaku D/Max 2500 diffractometer with a Cu rotating anode X-raysource working at 40 kV and 200 mA. The scanning rate was set as2�/min.

N2 adsorption-desorption isotherms at liquid nitrogen temper-ature were measured on a Quantachrome autosorb iQ analyzer.The samples were evacuated at 200 �C for 6 h before measurement.

The Pt content was measured on a PerkinElmer Optima 7300DVICP-OES spectrometer.

High-resolution transmission electron microscope (HRTEM)and scanning transmission electron microscope (STEM) imageswere recorded on a JEOL JEM-F200 multi-purpose electron micro-scope and a JEOL JEM-ARM300F transmission electron microscope.

X-ray photoelectron spectroscopy (XPS) was recorded on aThermofisher ESCALAB 250Xi instrument with a monochromaticAl Ka X-Ray source. The spectra were calibrated by adjusting C1s peak to 284.6 eV.

The temperature-programmed reduction with H2 (H2-TPR) andtemperature-programmed desorption of H2 (H2-TPD) were per-formed using a Quantachrome ChemStar TPx chemisorption ana-lyzer equipped with a TCD detector. For H2-TPR, the sample waspretreated in Ar at 120 �C for 1 h, then cooled to room temperature,followed by temperature-programmed reduction by 10% H2/Ar at aramping rate of 10 �C/min. For H2-TPD, the sample was heatedfrom room temperature to 300 �C at a ramping rate of 10 �C/minand held at 300 �C for 1 h, followed by cooling to 50 �C in a 10%H2/Ar mixture. After that, the carrier gas was switched to Ar toremove the weakly adsorbed H2. Then the temperature was ramp-ing at 10 �C/min for H2 desorption.

H2-D2 isotopic exchange experiment was performed using anAutoChem 2910 chemisorption analyzer (Micromeritics). The sam-ple was pretreated in He at 300 �C for 1 h, followed by cooling toroom temperature. Then the treatment gas was switched to mix-tures of H2 and D2. The temperature was ramping at 5 �C/min. Massspectrometer (OmniStar) was used to monitor the effluent gas.

n+ species as highly active sites in Pt/In2O3 catalysts for methanol synthesis0.06.018

https://doi.org/10.1016/j.jcat.2020.06.018

Table 1Reaction results of the catalysts at T = 300 �C, P = 2 MPa, and SV = 24,000 mL�h�1�gcat-1 .

Catalyst CO2 conversion (%) Product selectivity (%)

CO CH3OH CH4 CH3OCH3

In2O3 4.4 51.4 48.0 0.6 –0.03 wt.% Pt/In2O3 4.6 45.1 54.2 0.7 –0.13 wt.% Pt/In2O3 5.4 40.9 58.4 0.7 0.010.34 wt.% Pt/In2O3 6.1 42.6 56.8 0.6 0.020.58 wt.% Pt/In2O3 6.3 43.6 55.8 0.6 0.022.50 wt.% Pt/In2O3 8.3 58.6 41.0 0.3 0.01

Z. Han et al. / Journal of Catalysis xxx (xxxx) xxx 3

3. Results and discussion

3.1. CO2 hydrogenation on Pt/In2O3

A series of Pt/In2O3 catalysts containing 0.03–0.58 wt.% Pt wereprepared by co-precipitation. For comparison, a Pt/In2O3 catalystwith relatively high Pt loading (2.50 wt.%) was also prepared bywet impregnation. The reaction results at 300 �C and 2 MPa areshown in Fig. 1 and Table 1. Methanol and CO are the major prod-ucts with a small amount of CH4 (< 0.8%) for all the catalysts.Dimethyl ether (< 0.1%) is also detected when CO2 conversion isrelatively high, which is formed from the dehydration of methanol.For the Pt/In2O3 catalysts prepared by co-precipitation, the conver-sion of CO2 increases monotonically with Pt loading. The introduc-tion of Pt hardly changes the CH4 selectivity. In2O3 shows amoderate methanol selectivity (48.0%), which is increased to54.2% upon the introduction of 0.03 wt.% Pt. The highest methanolselectivity (58.4%) is achieved for 0.13 wt.% Pt/In2O3. Furtherincreasing the Pt content to 0.34 wt.% and 0.58 wt.% results in adrop of methanol selectivity to 56.8% and 55.8%, respectively. Butthe methanol selectivity of these two catalysts is still higher thanthat of In2O3. The 2.50 wt.% Pt/In2O3 used for comparison showsthe lowest methanol selectivity (41.0%), even lower than that ofIn2O3. The significant impact of Pt loading on the methanol selec-tivity may be related to the different chemical states of the Ptspecies.

Fig. 2 shows the influence of reaction temperature on the cat-alytic performance. For all the catalysts, the lowering of reactiontemperature leads to a decrease in CO2 conversion and an increasein methanol selectivity. The CO2 hydrogenation to methanol isexothermic, while RWGS reaction is endothermic [35]. Therefore,a lower temperature is favorable for methanol synthesis. It is note-worthy that the highest methanol selectivity (91.1%) is achievedfor 0.58 wt.% Pt/In2O3 at 220 �C, much higher than that of In2O3(72.2%).

The stability of the 0.58 wt.% Pt/In2O3 was further tested at300 �C and 4 MPa (Fig. 3). The stability test results of In2O3 are alsoshown in Fig. 3 for comparison. The relatively low CO2 conversionfor all the catalysts at the first 1 h is due to the instability of thereactor PID temperature controller at the beginning. The CO2 con-version of In2O3 decreases continuously, especially at the initialstage, indicating the deactivation of In2O3. The deactivation iscaused by the sintering of In2O3 due to the product water [29].The methanol selectivity does not change significantly. The CO2conversion of 0.58 wt.% Pt/In2O3 also exhibits a slight decrease at

Fig. 1. (a) CO2 conversion and (b) methanol and CO selectivity of the catalysts. All thSV = 24,000 mL�h�1�gcat-1 .


the initial 30 h but remains almost the same afterward, suggestingthat this catalyst is more stable than In2O3. Similar to In2O3, themethanol selectivity of 0.58 wt.% Pt/In2O3 remains almostunchanged for the time we tested.

The influence of space velocity (SV) was investigated using the0.58 wt.% Pt/In2O3 at 300 �C and 4 MPa (Fig. 4). The CO2 conversiondrops with the increase of SV while the methanol selectivityincreases. The methanol space-time yield (STY) increases withthe SV in the range we tested. The highest methanol space–timeyield of 0.76 g�h�1�gcat-1 was achieved for this catalyst at SV of54,000 mL�h�1�gcat-1 .

3.2. Characterization and discussion

Fig. 5 shows the XRD patterns of the as-prepared catalysts. ForIn2O3, all the diffraction peaks are assigned to cubic In2O3 (PDF No.89-4595). No diffraction peak belongs to the rhombohedral struc-ture is observed, indicating the presence of pure cubic phase ofIn2O3. The diffraction patterns of the as-prepared Pt/In2O3 are sim-ilar to In2O3. There is no diffraction peak belongs to the metallic Ptor Pt oxides phase, suggesting that the particle size of Pt species issmaller than the detection limit of XRD or the Pt species is atomi-cally dispersed in the In2O3 lattice. The average crystallite size ofthe as-prepared catalysts is estimated using the Scherrer equationand shown in Table S1. The as-prepared catalysts have similaraverage crystallite size (10–12 nm). The specific surface area ofthese catalysts is determined from N2 adsorption-desorption iso-therms. These catalysts have similar specific surface area (64.5–87.6 m2�g�1, Table S1).

The TEM images show that the as-prepared 0.58 wt.% Pt/In2O3contains agglomeration of crystals in diameter from ca. 7 nm to12 nm (Fig. 6a, b, and Fig. S1), in agreement with the average crys-tallite size estimated from XRD (Table S1). The lattice fringes indi-

e catalysts were tested under conditions of T = 300 �C, P = 2 MPa, and



Fig. 2. (a) CO2 conversion and (b) methanol selectivity at different reaction temperatures. All the catalysts were tested under conditions of P = 2 MPa andSV = 24,000 mL�h�1�gcat-1 .

Fig. 3. Stability of the catalysts under conditions of T = 300 �C, P = 4 MPa, and SV = 24,000 mL�h�1�gcat-1 .

Fig. 4. The influence of space velocity on the (a) CO2 conversion, methanol selectivity, and (b) methanol space–time yield for 0.58 wt.% Pt/In2O3 at T = 300 �C, P = 4 MPa.


cate d spacing of 0.29 nm, corresponding to the (222) planes ofcubic In2O3, suggesting that these particles are In2O3 nanocrystals.Atomic resolution STEM is used to characterize the dispersion of Pt


species. There are individual bright spots in the STEM images(Fig. 6b and Fig. S1), indicating that the Pt species are atomicallydispersed in In2O3. Carefully examination of different regions does



Fig. 5. XRD patterns of the as-prepared In2O3 and Pt/In2O3.


not show any evidence for the existence of Pt nanoparticles, whichshould be bright patches in the STEM images if they exist.

The used 0.58 wt.% Pt/In2O3 was also studied by TEM. After usedin CO2 hydrogenation, the 0.58 wt.% Pt/In2O3 remains to be anagglomeration of small crystals (Fig. 6d, e, and Fig. S2). However,the crystals become larger compared to the catalyst before reac-tion, implying the sintering of the catalyst during CO2 hydrogena-tion, which has also been proved by XRD (Fig. S6). The Ptnanoparticles in diameter of 1–2 nm can be observed in the STEMimages of the used 0.58 wt.% Pt/In2O3 (Fig. 6e and Fig. S2). These Ptnanoparticles come from the reduction and sintering of the atom-ically dispersed Pt species during reaction. It should be noted that

Fig. 6. (a, d) HRTEM and (b, e) STEM images of (a, b) as-prepared 0.58 wt.% Pt/In2O3 and (dnanoparticles are marked with arrows and circles, respectively, in the STEM images. Mshown on the right side of the STEM images. The yellow ball represents In2O3. Thenanoparticles, respectively.


there is still atomically dispersed Pt species left as revealed by thediscrete bright spots in the STEM images (Fig. 6e and Fig. S2),implying that part of the atomically dispersed Pt species is stableagainst sintering during CO2 hydrogenation. The TEM images ofused 0.13 wt.% Pt/In2O3 are shown in Fig. S3 for comparison. Ptnanoparticles with similar size (1–2 nm) are also observed, buttheir counts are less compared to the used 0.58 wt.% Pt/In2O3. SinceCO2 conversion increases while the methanol selectivity dropswhen the Pt content is increased from 0.13 wt.% to 0.58 wt.%(Fig. 1 and Table 1), the enhanced RWGS reaction should be relatedto the increasing amount of Pt nanoparticles. The enhancement ofRWGS reaction by Pt nanoparticles is further confirmed by the2.50 wt.% Pt/In2O3, which has much more Pt nanoparticles afterreaction based on TEM images (Fig. S4) and exhibits the highestCO selectivity.

The chemical states of surface elements were investigated byXPS. The strongest photoelectron peaks of Pt (Pt 4f peaks) overlapwith the In 4p peaks, so Pt 4d peaks are used for the identificationof the chemical states of Pt. The Pt 4d peaks are not observed forthe catalysts with Pt content lower than 0.34 wt.% (Fig. S7a) dueto the low content. For 0.34 wt.% Pt/In2O3 and 0.58 wt.% Pt/In2O3,the Pt 4d5/2 peak (Fig. 7) can be deconvoluted into two peaks locateat 315.3 eV and 317.0 eV, which belong to Pt2+ and Pt4+, respec-tively [36]. The possibility that Pt species forms separate PtO andPtO2 phase has been excluded based on XRD and TEM. Therefore,the atomically dispersed Pt species is in the cationic state insteadof the metallic state. These Ptn+ (Pt2+ and Pt4+) cations may replacethe In3+ ions in the In2O3 lattice or locate in the interstitial sites,bonding with the lattice O2� strongly. They may also locate onthe defect sites on the surface of In2O3. At first sight, the bindingenergy of Pt 4d5/2 peak for 2.50 wt.% Pt/In2O3 is higher than0.58 wt.% Pt/In2O3. This is caused by the different chemical states

, e) used 0.58 wt.% Pt/In2O3 after reaction. The atomically dispersed Pt species and Ptodels of the (c) as-prepared 0.58 wt.% Pt/In2O3 and (f) used 0.58 wt.% Pt/In2O3 arered dots and blue patches represent the atomically dispersed Pt species and Pt



Fig. 7. XPS Pt 4d spectra of the as-prepared catalysts and used catalysts afterreaction.


of Pt species. This peak can also be deconvoluted into two peaks at315.3 eV and 317.0 eV. The first one belongs to Pt2+, while the sec-ond one belongs to Pt4+.

The Pt 4d5/2 peaks of used 0.58 wt.% and 2.50 wt.% Pt/In2O3 bothshift to lower binding energy, confirming the partial reduction ofPtn+ species (Pt2+ and Pt4+) during the reaction. The deconvolutionresults (Table 2) show that about 12.2% of the Ptn+ species in0.58 wt.% Pt/In2O3 is reduced to Pt0 while others are still in atom-ically dispersed Ptn+ state. For 2.50 wt.% Pt/In2O3, relatively morePt0 (47.9%) is formed compared to 0.58 wt.% Pt/In2O3. It shouldbe noted that the used samples were exposed to air before theXPS measurement. Thus, the ratio of Ptn+ species for the used cat-alysts may be overestimated by XPS due to the possible partial oxi-dation of metallic Pt. However, the existence of atomicallydispersed Ptn+ species in the used 0.58 wt.% Pt/In2O3 has alreadybeen demonstrated by atomic resolution STEM. The comparisonof TEM images between used 0.58 wt.% and 2.50 wt.% Pt/In2O3 alsoproves that the later sample has more Pt nanoparticles. Consider-ing that the methanol selectivity is enhanced when a small amountof Pt is introduced into In2O3, but drops when the Pt loading ishigher (Fig. 1 and Table 1), it is reasonable to propose that theatomically dispersed Ptn+ species is responsible for the increasedmethanol selectivity while Pt nanoparticles mainly boost theRWGS reaction.

To further prove that Pt nanoparticles are mainly responsiblefor the enhanced RWGS reaction, we prepared a Pt/SiO2 catalystwith Pt loading of 0.65 wt.% by wet impregnation. The catalystwas tested at 300 �C and 2 MPa (reaction conditions same to theconditions used in Fig. 1 and Table 1). This catalyst is active inCO2 hydrogenation and exhibits very high selectivity to CO (>99.9%) at a CO2 conversion of 1.2%. TEM images of the used catalyst(Fig. S5) show plenty of Pt nanoparticles with a wide size distribu-

Table 2Deconvolution results of the Pt 4d5/2 XPS peaks.

Sample Atomic concentration (%)

Pt0

(314.2 eV)Pt2+

(315.3 eV)Pt4+

(317.0 eV)

2.50 wt.% Pt/In2O3 afterreaction

47.9 29.4 22.7

2.50 wt.% Pt/In2O3 0 42.7 57.30.58 wt.% Pt/In2O3 after

reaction12.2 85.8 2.0

0.58 wt.% Pt/In2O3 0 78.9 21.10.34 wt.% Pt/In2O3 0 92.7 7.3


tion range (1–6 nm). The Pt 4f XPS peaks of the used 0.65 wt.% Pt/SiO2 (Fig. S10) at 70.6 eV (4f7/2) and 73.9 eV (4f5/2) also clearly indi-cate that the Pt species in the used 0.65 wt.% Pt/SiO2 is in themetallic state [37]. Therefore, these results lead us to the conclu-sion that Pt nanoparticles mainly boost the RWGS reaction.

The binding energy of In 3d5/2 peak (Fig. S7b) for In2O3 is443.8 eV, implying the +3 oxidation state of In [38]. After addingPt, the In 3d XPS peaks shift to higher binding energy. This shiftis caused by the higher electronegativity of Ptn+ (1.513 for Pt2+

and 1.880 for Pt4+) than In3+ (1.445) [39]. When the In3+ ions inthe In2O3 lattice are replaced by Ptn+, part of the In-O-In structureis replaced by Pt-O-In structure. Electrons transfer from In to Ptthrough O partially due to the higher electronegativity of Ptn+.The In3+ ions become more electron deficient. Therefore, the 3dXPS peaks of In shift to higher binding energy. The shift proves thatthe interaction between atomically dispersed Ptn+ species andIn2O3 lattice is strong, which may be important in the stabilizationof the Ptn+ species during CO2 hydrogenation. The O 1s peaks of allthe catalysts are asymmetric (Fig. S7c). This is due to the presenceof surface OH groups and oxygen vacancies [40].

The relative surface atomic ratio of Pt and In was calculatedfrom XPS and shown in Table S2. The surface Pt content increasesslightly for the 0.58 wt.% Pt/In2O3 after reaction, which is caused bythe reduction and sintering of part of the atomically dispersed Ptn+

species. On the contrary, the surface Pt content for 2.50 wt.% Pt/In2O3 decreases after reaction. Since this catalyst is prepared byimpregnation, most of the Pt species locate on the surface for theas-prepared catalyst and are reduced to Pt nanoparticles duringreaction. The decrease of surface Pt content is due to the migrationof In2O3 over Pt nanoparticles, which is widely observed for redu-cible oxides caused by the strong metal support interaction.

H2-TPR profiles of the as-prepared catalysts are shown in Fig. 8aand Fig. S8. In2O3 gives small reduction peaks which start at ca.120 �C, originating from the creation of surface oxygen vacanciesby H2 reduction. The strong H2 consumption starts at ca. 430 �C(Fig. S8) is attributed to the reduction of bulk In2O3. The reductionpeak at ca. 141 �C becomes more significant with the increase of Ptloading for 0.03–0.58 wt.% Pt/In2O3 and shifts towards low temper-ature slightly. This enlarged peak may be attributed to the creationof more oxygen vacancies with more Ptn+ species. For 0.34 wt.% and0.58 wt.% Pt/In2O3, a new reduction peak appears at ca. 110 �C,which is probably ascribed to the reduction of Ptn+ species on thesurface of In2O3 and not in the In2O3 lattice. Since this part ofPtn+ ions locate on the surface of In2O3, they are easier to bereduced compared to the Ptn+ ions in the In2O3 lattice. The reduc-tion of these species results in the formation of Pt nanoparticles. Itshould be noted that although H2-TPR shows the reduction of partof the Ptn+ species below reaction temperature, the reduction of theatomically dispersed Ptn+ species in the In2O3 lattice should beharder than this kind of species on the surface due to the stronginteraction with In2O3 lattice, which has been proved by XPS. Onthe other hand, the reactant CO2 and the product H2O can serveas oxidants for the stabilization of Ptn+ species in the oxidationstate. The oxidation effect of CO2 [41] and H2O [42] under workingconditions has been documented in other catalytic systems. Forinstance, small metallic Co nanoparticles are not stable and proneto be oxidized by H2O under Fischer-Tropsch synthesis conditions[43]. Metallic Ni in Ni/SiO2 can be partially oxidized to Ni2+ by CO2at reaction temperature [41]. In situ EXAFS also demonstrates thatpart of the metallic Cu is oxidized to Cu+ and Cu2+ during CO2hydrogenation on Cu/ZrO2 [44]. Therefore, not all the Ptn+ speciesare reduced to Pt0 state during CO2 hydrogenation, as alreadyproved by XPS and TEM.

The H2-D2 isotopic exchange experiment is used to probe thefunction of Pt in the activation of H2 (Fig. 8b and Fig. S9). In2O3shows relatively weak ability to catalyze the isotopic exchange of



Fig. 8. (a) H2-TPR profiles, (b) H2-D2 exchange results at 100 �C, and (c) H2-TPD profiles of the catalysts.


H2 and D2 at 100 �C with a D2 conversion of 3%. The D2 conversionis enhanced to 7% upon the addition of only 0.03 wt.% Pt. The 0.13–0.58 wt.% Pt/In2O3 are much more active for the H2 activation, andall achieve exchange equilibrium at 100 �C. H2-TPR proves that thereduction of Ptn+ species does not happen at 100 �C. Thus, theenhanced ability to activate H2 can be ascribed to the atomicallydispersed Ptn+ species. A similar conclusion that the atomically dis-persed Ptn+ species can facilitate the H2 activation has also beenreported for other Pt-based single-atom catalysts such as Pt onphosphomolybdic acid-modified active carbon (Pt/PMA/C) [45],Pt/m-Al2O3 [46], and Pt/FeOx [47]. The Pt single-atom species inPt/PMA/C is anchored on the four-fold hollow site on phospho-molybdic acid by coordinating to oxygen atoms and positivelycharged by electron transfer from Pt to PMA [45]. This catalystexhibits substantial activity in the hydrogenation of nitrobenzeneand cyclohexanone. Similar to Pt/PMA/C, the Pt single-atom spe-cies in Pt/m-Al2O3 [46] and Pt/FeOx [47] coordinates with oxygenatoms on the oxide supports and is in oxidized state, which is alsoactive in a variety of hydrogenation reactions.

Fig. 8c shows the H2-TPD profiles of these catalysts. In2O3 andPt/In2O3 exhibit desorption peaks at temperature range between120 �C and 300 �C. In2O3 has two desorption peaks, one main peakcentered at ca. 257 �C with a shoulder peak centered at ca. 172 �C.These two peaks belong to H2 adsorbed on different sites. For0.03 wt.% Pt/In2O3, there is only one desorption peak centered atca. 257 �C. When the Pt content is increased to 0.13 wt.%, this peak


shifts to 242 �C. Further increasing the Pt content makes this peakshift towards low temperature. The shift suggests that H2 desorbsmore easily on Pt/In2O3 compared to In2O3. The change of H2-TPDprofiles indicates that the introduction of Pt greatly changes theadsorption strength of the adsorbed hydrogen species.

During CO2 hydrogenation, the surface of In2O3 provides anoxygen vacancy containing In3O5 structure, serving as a frustratedLewis pair like structure for the heterolytic dissociation of H2 [29].The formed hydride species (Hd-) transfers to the chemisorbed CO2,results in the formation of formate (HCOO�) intermediate species,which can be further hydrogenated to methanol [29]. The intro-duction of Pt into In2O3 improves both the activity and the metha-nol selectivity of the catalysts. When Pt was introduced into In2O3by co-precipitation, the positively charged Pt species can be atom-ically dispersed into the In2O3 lattice. During the reaction, part ofthe Pt species is reduced and sintered, forming Pt nanoparticles.Considering the stability of the 0.58 wt.% Pt/In2O3, we propose thatmost of these Pt nanoparticles are generated at the very initialstage of the reaction. Meanwhile, there are still a significantamount of atomically dispersed Ptn+ species remained in theIn2O3 lattice, which are stabilized by the strong interaction withIn2O3 lattice and the oxidation effect of CO2 and product H2O.The atomically dispersed Ptn+ species and Pt nanoparticles are bothactive in H2 splitting. H2 activated on these sites can spill over tothe In2O3, assisting in the creation of more oxygen vacancies,which are indispensable in CO2 activation [29]. At the same time,




the atomically dispersed Ptn+ species can act as a Lewis acid siteand participates in the heterolytic splitting of H2. This is similarto the heterolytic splitting of H2 on the oxygen vacancy containingIn3O5 structure, which may be crucial for the high methanol selec-tivity of In2O3 [29]. Therefore, the methanol selectivity is improvedby the atomically dispersed Ptn+ species. On the other hand, theformed Pt nanoparticles predominately assist in the homolyticcleavage of H2 and the formation of CO, which is proved by thecomparison of the reaction results between 0.13 wt.% and0.58 wt.% Pt/In2O3 and the high CO selectivity of the reference2.50 wt.% Pt/In2O3 and 0.65 wt.% Pt/SiO2.

4. Conclusions

In this work, we introduced Pt into In2O3 to investigate the roleof Pt species in CO2 hydrogenation. The Pt atoms can be embeddedin the lattice of In2O3 as atomically dispersed Ptn+ species. DuringCO2 hydrogenation, part of the positively charged Pt species wasreduced and sintered, forming Pt nanoparticles, while othersremained in the atomically dispersed, positively charged state.The remaining atomically dispersed Ptn+ species and the formedPt nanoparticles are both active in the activation of H2 and helpto create more oxygen vacancies, on which CO2 is activated. Atthe same time, the atomically dispersed Ptn+ species that is stableunder reaction conditions acts as a Lewis acid site to promote theheterolytic dissociation of H2 and helps in the methanol formation,while Pt nanoparticles mainly boost the RWGS reaction.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgments

This work was supported by grants from National Key R&D Pro-gram of China (No. 2017YFB0702800), National Natural ScienceFoundation of China (No. 21802139), and Project of Key ResearchPlan of Ningxia (2019BDE03003). M. R. Li and N. Ta helped in theelectron microscopy measurements.

Appendix A. Supplementary material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jcat.2020.06.018.

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Atomically dispersed Ptn+ species as highly active sites in Pt/In2O3 catalysts for methanol synthesis from CO2 hydrogenation1 Introduction2 Experimental section2.1 Catalyst preparation2.2 Catalyst evaluation2.3 Catalyst characterization

3 Results and discussion3.1 CO2 hydrogenation on Pt/In2O33.2 Characterization and discussion

4 ConclusionsDeclaration of Competing InterestAcknowledgmentsAppendix A Supplementary materialReferences

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